PARIS — The scandal over horse meat in the European food chain widened Thursday from a case of mislabeling to one of food safety as public health authorities in Britain said that a powerful equine painkiller, potentially harmful to human health, “may have entered the food chain” in France.

British officials sought to reassure the public that the drug — phenylbutazone, or bute, an anti-inflammatory used commonly on lame horses — was found only in trace amounts in a small number of British horse carcasses. Out of 206 carcasses, 8 tested positive for bute, and just 6 of those carcasses were exported to France.

The drug is also used to treat arthritis in humans. Very large doses can cause a potentially fatal blood disorder, aplastic anemia, in which the bone marrow fails to produce enough blood cells.

Even before the discovery, the scandal had plunged the European food industry into crisis. Frozen foods, including hamburger, lasagna, spaghetti Bolognese and moussaka, have been withdrawn from supermarket freezers in Britain, Ireland, Sweden, France and Germany.

But the positive tests for the equine drug have raised new concerns, even in countries like France, where eating horse meat is more acceptable than in Britain, where it is taboo. Unlike cattle, which are raised for slaughter in controlled conditions, the lame horses or former plow animals that are sometimes killed for consumption do not carry the same guarantees of origin or quality.

“Unscrupulous criminal elements have been profiting from these unfortunate animals by exploiting a hopelessly flawed horse passport scheme, lax animal export controls at ports, and slaughterhouses willing to compromise animal welfare and public safety for a quick profit,” said David Wilson, the spokesman for the Ulster Society for the Prevention of Cruelty to Animals, in Northern Ireland. The discovery of suspect frozen beef hamburgers in Ireland set off the crisis.

“Many horses that are either valueless or in suffering are being corralled in remote farmyards and packed into vehicles for transport to abattoirs in Ireland and the U.K.,” he said. “After slaughter, many are processed and find their way effortlessly into the European food chain.”

Health experts said that a human would have to eat 500 or 600 burgers a day of 100 percent horse meat to be adversely affected by the drug at the level found in the carcasses.

The police investigating allegations that horse meat was mislabeled as beef said Thursday that they had arrested two men in Wales and another man in West Yorkshire, England, on Tuesday and were holding them on suspicion of fraud.

Checking food safety “from farm to fork” in Europe has been a persistent worry for two decades, since the crisis of mad cow disease in Britain threw the Continent into a crisis and prompted a ban on British beef exports to Europe and the United States.

The current scandal has laid bare a supply chain for meat so murky and complex that officials and experts say it is easily susceptible to fraud and manipulation by organized crime. The convoluted route from slaughterhouse to plate has highlighted a weak system of accountability in a vast single market of almost 500 million people, where retailers use layers of meat traders to find the cheapest deals and no regular tests are conducted to authenticate meat products.

“In a cash-rich business like the meat industry, there is ample opportunity for criminals to bribe bureaucrats,” said Misha Glenny, an expert on organized crime in Europe. “Market opportunities create criminal opportunities.”

After days of investigation, the French authorities announced on Thursday that they suspected fraud at a French company, Spanghero, which supplied meat — including large quantities of horse meat — to Comigel, a French-owned company whose Luxembourg factory used the meat to make frozen meals sold in supermarkets in 16 countries across Europe, including Britain and Sweden. They said they would suspend Spanghero’s operating license. The company denied any wrongdoing.

The French investigators said that the meat Spanghero sold to Comigel had originated in two Romanian slaughterhouses, in a shipment that they said showed signs of containing horse meat. It was then bought by a Dutch food trader who sold it to a Cypriot-registered food trader who sent it to Spanghero.

“The investigation shows that Spanghero knew that the meat labeled as beef could be horse,” Benoît Hamon, France’s junior minister for social economy, said Thursday. “There was a strong suspicion.” Mr. Hamon added that the fraudulent meat sales had been going on for several months, and involved 28 companies and 13 countries.

Mr. Hamon said that Comigel should have suspected something was amiss. While Comigel maintained it did not realize the meat was not beef, he said, anyone in the business should have been able to tell the difference.

In a statement, Spanghero said it was the victim, not the perpetrator, of the fraud. “Spanghero confirms having placed an order for beef, having been led to believe it received beef, and having sold back what it thought was beef, properly labeled as such, in line with European and French regulations.”

Chris Elliott, director of the Institute for Global Food Security at Queen’s University, Belfast, said the complex supply chain was vulnerable to fraud since the contents of meat shipments were not routinely verified.

“There are multiple contracts in these supply chains, and the contracts are very tight,” he said. “But there are absolutely no checks. It is really done on trust that the piece of paper that says that you have five tons of beef is beef. There is no verification of the audit trail.”

Spanghero said earlier that it would sue its Romanian suppliers, though French authorities say those suppliers acted in good faith and properly labeled their shipment as horse meat.

“We have respected all of the labeling laws,” Iulian Cazacut, the director of the meatpacking plant, Doly-Com, in Romania’s impoverished northeast, said in an interview. “I am not responsible for the mistakes of others.”

Mr. Cazacut said the company, which processed 427 tons of horse last year, bought horses, no longer fit for work, from local farmers for around 60 cents a pound, far less than the approximately $2.50 a pound fetched by beef.

Dan Bilefsky reported from Paris, and Stephen Castle from London. Reporting was contributed by George Calin from Bucharest, Romania; Andrew Higgins from Brussels; Douglas Dalby from Dublin; and Melissa Eddy from Berlin.

A version of this article appears in print on February 15, 2013, on page A6 of the New York edition with the headline: Britain Says Equine Drug May Be in Food Chain. Order Reprints|Today's Paper|Subscribe